Network units, such as switches and routers which include a multiplicity of ports for connection to other units or users are commonly made with a fixed number of ports in order to achieve efficiency of manufacture. It is well known to ‘stack’ such units, by which is meant their connection by what is known as a cascade connection, which enables them not only to receive and forward packets from their own ports but also to forward packets to other units in the stack where destination ports are on a unit other than the unit which first received the packet. Packets which are passed between the units for this purpose travel on a cascade connection, which normally comprises a data path (for the packets) and a control path which enables the units to exchange control and status information for a variety of purposes. These paths may be physically separate but need not be. Physically the units may be stackable, though the physical aspect of stacking is not an essential feature of the present invention. Known stacking arrangements are disclosed and exemplified by the Super Stack 3 Switch 3300 made by 3Com Corporation of Santa Clara, Calif. and the Switch 4400 of the same Corporation.
Although the facility of stacking has a substantial number of operational benefits, there are difficulties which may offset those benefits.
For example, since all packets that are received by any unit in the stack and require forwarding from another unit must travel by way of the cascade, the requirement for bandwidth in the cascade connection can be very substantial. It is desirable in almost all circumstances to minimise the usage of bandwidth on the cascade.
Two well-known network functions are ‘bridging’ and ‘routing’. The former term, as well as the term ‘bridge’, is normally used to signify the receiving and forwarding of addressed data packets in accordance with media access control addresses (layer 2 addresses).
Routing is used to signify communication between different sub-nets or networks. Although media access control addresses are employed to convey a packet on each hop of a journey, routers are employed to determine a multiple-hop route for a packet. For this purpose it has recourse to a number of well known resources, such as a router information protocol or the like by means of which it can build ‘directories’ or routing tables.
It is known to provide a stack of switch units which have routing enabled in one unit only, the routing functions if any in the other units being disabled. Such a scheme requires all traffic that requires routing and not received at or forwarded from the routing unit to traverse the cascade twice, and also heavily loads the single active routing entity.